1. Field of the Invention
The present invention relates to infrared radiation detectors and, more particularly, to small bandgap semiconductor infrared detectors and methods of fabrication.
2. Description of the Related Art
Detection of infrared radiation emitted by warm bodies provides an important method for night vision (perception without visible light). Infrared detectors are of various types and include small bandgap semiconductors structured as photodiodes or photocapacitors. Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as the photosensitive semiconductor in such detectors. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te has a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors.
An-infrared imager incorporating an array of MIS photocapacitor detectors in Hg.sub.1-x Cd.sub.x Te is disclosed in U.S. Pat. No. 4,684,812 (Tew and Lewis). FIGS. 1a-b are cross sectional elevation and plan views of a single photocapacitor and illustrate the anodic oxide passivation of the Hg.sub.1-x Cd.sub.x Te. Anodic oxidation of Hg.sub.1-x Cd.sub.x Te provides a passivation layer made of primarily HgTeO.sub.3, CdTeO.sub.3, and TeO.sub.2 on the surface of the Hg.sub.1-x Cd.sub.x Te (typically the layer is grown to a thickness of 700 .ANG.), and this oxide passivation layer typically includes substantial trapped positive charge with density of roughly 1.times.10.sup.12 /cm.sup.2. The trapped charge acts as a channel stop in that it accumulates electrons at the oxide Hg.sub.1-x Cd.sub.x Te interface and deters inversion in n type Hg.sub.1-x Cd.sub.x Te. This achieves maximum usable potential well capacity and integration time.
However, the anodic oxide passivation layer is thermodynamically unstable and sulfide passivation layers (primarily CdS plus some HgS) are replacing oxide passivation layers; see U.S. Pat. No. 4,632,886 (Teherani and Simmons). Sulfide passivation layers have the drawback of not incorporating substantial positive fixed charger, and various approaches have been used to control surface currents. One approach is to etch away the sulfide passivation layer in channel stop areas and grow anodic oxide to regain the fixed positive charge, but this has the problem of extra wet processing steps. Alternatively, a field plate may be incorporated into the imager and biased to induce a channel stop, but this has the problems of a more complicated structure and processing sequence.
Infrared detectors based on photodiodes in Hg.sub.1-x Cd.sub.x Te require formation of p-n junctions in Hg.sub.1-x Cd.sub.x Te, and such junctions are frequently made by ion implanting species such as boron into p type Hg.sub.1-x Cd.sub.x Te. Prior to annealing, the implanted region has strong n type doping even for very small implanted doses of each kind of implanted ion (donor, acceptor, inert) and the carrier concentration increases with implant dose but rapidly reaches a saturation in sheet donor concentration of 2.times.10.sup.14 /cm.sup.2. See, G. Destefanis, 86 J.Crystal Growth 700 (1988) at 705.